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58',                        METALL'UKGY OP IRON AND STEEL.
- 1^3) Temperature of the furnace:                              .
, The elimination of sulphur is assisted by a high temperature; but temperature alone is not sufficient, for with a silicious slag the iron may be high in sulphur, even" though the furnace be hot; but any particular slag will caxiy more sulphur with a not furnace! than when' the hearth is cold. Hence,, a slag which is quite suitable for a hot furnace must be made more limey if the temperature is'> reduced, or the iron will be higher in sulphur. On the other hand,' a limey slag, will not run fluid in a cool furnace, so that the furnace-man is held between narrow limits.
r, It is essential in practice, in addition to the removal of sulphury that the content of silicon in the iron be' regulated.   This can be done by a proper control of the temperature and of the slag.    A" rise in temperature gives higher silicon in the iron, -because the coke has then a greater affinity for the oxygen of the silica and sets" free; the silicon.   On the other hand, an increase in the amount of lim,e gives lower silicon, because the silica is needed by the lime to: form a slag.   The amount of silica present has something to do with the result; a furnace working on impure ores may handle" twice the weight of silica per ton of .iron that is carried by a furnace on a rich burden, and make twice the weight of slag, and with this greater exposure- of silica to reduction the tendency "will be toward a, higher silicon in the iron.   The control of the silicon": and the control of the sulphur constitute two problems, quite sepa-. rate from one another, and yet closely related.   The determihingj conditions may be grouped under four general divisions: ,".-(!) An iron with high silicon and low sulphur is made by run-' ning the furnace at a high temperature with a slag sufficiently basic tp. hold the sulphur, but not basic enough to keep silicon from be-1 ing reduced.                                                                                 :
(2)  An iron with low silicon and low sulphur is made by using > a lower ^temperature with a .somewhat more basic slag, or-a high temperature with a much more basic slag.
(3)   An iron with low silicon and high sulphur is made by using a low temperature with a slag not sufficiently basic.
,, "(-4) An iron with-high silicon and high" sulphur is made by using a high temperature with a slag not sufficiently basic.               . '.
.The presence of manganese complicates the metallurgy of the fur-" nace, but does not change any of the foregoing laws. An acid slag